What depth is the trough ?
It would have to be fairly deep to allow the plants roots to establish properly, i'd think a depth of 12" minimum, especially for Clematis and Roses, so too Lathyrus i believe.

I have my Lathyrus growing up an obelisk, so like you, it's a very 'open' area to grow up .... mine's doing fine up it so i wouldn't think your wide trellis' nature would be a problem.

Which Clematis do you have there ?

Agree with Louise - its not the trellis, its the trough that's the problem - if its one of those ones you can buy ready made, they're never deep enough for climbers anyway, I can never understand why they sell them with a trellis, unless its just for a moveable privacy screen.

Thank you to Louise and Bamboo - the trough is about 9 inches planting depth. I dont know the name of the clematis, one is a small white flower and the other quite a large purple one.
Do you have any suggestions for something to climb up the trellis. It is the end panel of our 2 1/2 foot fence nearest to the house, which borders our raised patio.
Thanks for your help so far.

I can't think of anything permanent that would grow up the trellis from a depth of 9 inches except Ivy,possibly, but you could try temporary summer climbers such as climbing nasturtium, they might tolerate it.

thanks